1. Field of the Invention
The present invention relates to the anode plate of a flat display screen. It more particularly relates to the realization of connections of phosphor elements of an anode for color screens, such as color screens including microtips.
2. Discussion of the Related Art
FIG. 1 represents a portion of a flat display screen with microtips of the type to which the invention applies.
Such microtip screens are mainly constituted by a cathode plate 1 including microtips 2 and by a gate 3 provided with holes 4 corresponding to the positions of the microtips 2. Cathode 1 is disposed so as to face a cathodo-luminescent anode 5, formed on a glass substrate 6 that constitutes the screen surface.
The operation and a detailed structure of such microtip screens are described in U.S. Pat. No. 4,940,916 assigned to Commissariat a l'Energie Atomique.
The cathode 1 is arranged in columns and is constituted, onto a glass substrate 10, of cathode conductors arranged in meshes from a conductive layer. The microtips 2 are disposed onto a resistive layer 11 that is deposited onto the cathode conductors and are disposed inside meshes defined by the cathode conductors. FIG. 1 partially represents the inside of a mesh, without the cathode conductors. The cathode 1 is associated with the gate 3 which is arranged in rows. The intersection of a row of gate 3 with a column of cathode 1 defines a pixel.
This device uses the electric field generated between the cathode 1 and gate 3 so that electrons are extracted from microtips 2 toward phosphors 7 of anode 5. In the case of a color screen, the anode 5 is provided with alternate phosphor strips 7r, 7g, 7b, each corresponding to a color (red, green, blue). The strips are separated one from the other by an insulating materiel 8. The phosphors 7 are deposited onto electrodes 9, which are constituted by corresponding strips of a transparent conductive layer such as indium-tin oxide (ITO). Each group of red, green, blue strips is alternatively biased with respect to cathode 1 so that the electrons extracted from the microtips 2 of one pixel of the cathode/gate are alternatively directed toward the facing phosphor elements 7 of each color.
The selection of the phosphor element 7 (the phosphor element 7g in FIG. 1) that should be bombarded by electrons from the microtips 2 of cathode 1 requires to selectively control the biasing of the phosphor elements 7 of anode 5, for each color.
FIG. 2 schematically illustrates an anode structure for a conventional color screen. FIG. 2 is a partial view of an anode 5 fabricated according to known techniques. The strips 9 of the anode electrodes, corresponding to phosphor elements 7 of a same color deposited on the substrate 6, are interconnected outside the useful surface of the screen to be connected to a control system (not shown). Two interconnection tracks 12 and 13, respectively corresponding to anode electrodes 9g and 9b, are realized for two of the three colors of the phosphors (for example 7g and 7b). An insulation layer 14 (represented in dot-and-dash lines in FIG. 2) is deposited over the interconnection track 13. A third interconnection track 15 is connected, through conductors 16 deposited over the insulation layer 14, to the anode electrode strips 9r designed for the phosphors 7r of the third color.
Generally, the rows of gate 3 are sequentially biased to a voltage of approximately 80 volts whereas the phosphor strips (for example 7g in FIG. 1) to be excited are biased at a voltage of approximately 400 volts, the remaining strips (for example 7r and 7b in FIG. 1) being at a zero voltage. The columns of cathode 1, whose voltage represents for each row of the gate 5 the brightness of the pixel defined by the intersection of the column of the cathode and of the row of gate 5 in the considered color, are set to respective voltages between a maximum emission voltage and a non-emission voltage (for example 0 and 30 volts, respectively).
The selection of the bias voltages is associated with the characteristics of the phosphors 7 and microtips 2.
Conventionally, below a 50-V voltage difference between the cathode and the gate, no electron emission occurs and the maximum emission corresponds to a 80-V voltage difference.
The voltage difference between the anode and the cathode is associated with the distance between the electrodes. A maximum voltage difference is desired for the screen brightness, which involves that the distance separating the electrodes should be as large as possible. However, the structure of the inter-electrodes gap, which includes spacers that may generate shadow areas in the screen if they are oversized, prevents this gap between the electrodes from being increased. Therefore, the distance separating the electrodes of a conventional screen is approximately 0.2 mm. This requires the selection of an anode/cathode voltage that is critical because of the possible formation of electric arcs. Destructive electric arcs can occur due to a possible irregularity of the distance separating each microtip or the gate layer from the phosphor elements of the anode. Furthermore, such irregularities are unavoidable by virtue of the small size and of the manufacturing process of the anode and of the cathode-gate.
On the cathode side, the resistive layer 11 makes it possible to limit the formation of destructive short-circuits between the microtips and the gate.
However, on the anode side, electric arcs may occur between the gate 3 and some of the phosphor elements 7 of the anode that are biased to draw the electrons from the microtips 2 (for example, the phosphor elements 7g in FIG. 1). Electric arcs can also occur between two adjacent phosphor strips (for example 7g and 7r in FIG. 1) because of the voltage difference between these two strips.